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Comment by exhilaration | Hacker News Reader

Why can't you see any stars in these pictures or the video they posted? I'd expect to see thousands of stars.

Because they choose not to take photos of the surrounding stars, and those that are visible they either photoshop out or process the images in such a way that they're not visible.

There's a sibling comment saying it's due to the exposure, that's wrong. That's not how Hubble works, it's not just a giant digital camera in orbit. The "photo" you're looking at is a composite from tens or hundreds of other images, so to start with most of the pixels you're looking at are just black filler.

You can see the black and white raw images at http://hst.esac.esa.int/ehst/#home search for "SATURN" under "Solar System Objects" and sort by date. A lot of the Hubble images from June 20th, 2019 have stars, including some visible between the rings and Saturn. They were just edited out because showing them wasn't the point.

Is there a good explanation/article about how they take/process these images? I would have liked it if they could have included a bit of text about why certain items were not in the picture/removed.

Are those stars? The “iduy37thq” image [0], for example, has various star-like points of light on top of the planet itself in addition to appearing in the background.

[0] http://hst.esac.esa.int/ehst-sl-server/servlet/data-action?R...

This look like the noise I get from my camera when I take very long exposures (> 10 minutes). The sensor starts heating up and producing these dots.

No they're probably not (I'm an astronomer). They're likely cosmic rays. When I've used Hubble data, then the processing uses multiple images to remove the cosmic rays (which aren't in common between images).

I'd love to understand how those bright dots and lines (so, cosmic rays?) appear on these images.

They're just high energy particles that interact with the CCD cameras typically used for astronomy. You get a bright point or particle track, as the particle interacts with the CCD producing electrons. This is worse for space telescopes, as there are more cosmic rays in space. For some simple information see: http://spiff.rit.edu/classes/phys373/lectures/cosmic_rays/co...

I'm an astronomer, and I think you're probably wrong. It's mostly down to exposure. Those little dots you're seeing in the images are likely cosmic rays. Those are removed by combining multiple images, as cosmic rays don't appear in the same places.

The pixel-sized dots are cosmic rays, those are obviously artifacts as some of them can be seen in front of Saturn.

I'm referring to objects you can consistently see between different observations. E.g. this one around 1/2 a planet width from the pole you can't see observed from 2018-06-06 14:35:05 until 2018-06-06 14:46:35:

http://hst.esac.esa.int/ehst-sl-server/servlet/data-action?R...

Although looking again perhaps that's a moon of Saturn's rather than a particularly bright star.

Not an astronomer, just an interested amateur. The few times I've been curious enough to look at the raw data behind these images I've found background celestial objects that are definitely edited out. That's understandable, the point is to show e.g. Saturn and not whatever background objects happened to be there at the time Hubble took the photo.

I'm just pointing out it's not all due to the light exposure.

The exposure is adjusted for Saturn which is much brighter than the stars.

Same thing that people ask about the Moon landing photos. Where are the stars?

Thank you, that makes perfect sense. I would still love to see what it looks like with the naked eye.

That said, it would be amazing to see HDR images of these stellar bodies.

Here's a picture of some of the light even HDR wouldn't likely uncover:

https://www.esa.int/spaceinimages/Images/2007/07/The_constel...

EDIT: A larger version of the image, with an equally huge url: https://m.esa.int/var/esa/storage/images/esa_multimedia/imag...

HDR and wide-spectrum are extensions along different measurement axes.

Similarly, fake-moon-landing conspiracists don't seem to understand the large magnitude of differences between heavily objects. Sunlight is typically so much brighter than stars such that the exposures best used in sunlight will wash out stars (make their images be swamped by sensor noise). This happens even as far out as Saturn, which gets much less sun than our moon. Thus the "no stars in photos" argument about Apollo holds no water.

Even in a backyard telescope, if you look at Saturn, its brightness will wash out all but the very brightest of nearby stars because your eye adjusts for Saturn's brightness, not star brightness. (One nearby "star" you may see is actually the moon Titan.)

The New Horizons probe did pick up star images in Ultima-Thule images, if you look at the raw exposures, but Ultima-Thule is really far away from the sun, beyond Pluto. And it's a dark body, requiring sensitive camera settings. Note that some raw exposures from some probes will show streaks that may be mistaken for stars, but are actually cosmic rays. They are usually digitally removed from final versions.

The boundary where planetary exposures will start to clearly show stars is roughly around Neptune's or Pluto's orbit. But processing may have to remove them because time-lapse and/or multiple exposures may be needed to properly capture dim planets and rocky bodies that far out. Any stars caught would otherwise look smeared or smugdy.

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